The invention relates to aqueous dispersions of hydrophilic polymers. These dispersions are obtained by polymerization in an aqueous solution of a mixture of monomers in the presence of a salt and/or of a polymer known as a stabilizer. They are also referred to as water-in-water dispersions.
Hydrophilic polymers are known to fulfill numerous functions, among the most important being their use as flocculant, as coagulant, as stabilizer, as thickener and as buoyancy agent. They are used in various forms and in various industrial sectors such as water treatment (urban, industrial and residual waters), paper and cardboard sheet manufacture, the petroleum industry (drilling, assisted recovery or EOR), and the processing of ores and various minerals such as fillers and pigments.
The polymerization technique leading to aqueous dispersions of hydrophilic polymers has been developed for many years.
This technique consists in performing the polymerization of a monomer or of a mixture of monomers in water containing a salt and/or other chemical agents such as dispersants in solution or in dispersion. Since the hydrophilic polymer formed during the polymerization is not soluble in the saline medium and/or contains dispersants, it precipitates when it reaches a sufficiently high molecular weight. At the end of the polymerization, a liquid dispersion of polymer particles suspended in the aqueous mixture is obtained.
This technique makes it possible very rapidly (within a few seconds) to dissolve the polymer in water during its use.
In U.S. Pat. No. 3,658,772, Dow Chemical describes dispersions obtained by copolymerization of acrylic acid and of another monomer in an aqueous solution containing the inorganic salts. The dispersion obtained is characterized by a highly acidic pH. The polymerization time is between 16 and 22 hours.
Many other patents have since been filed. The Hymo U.S. Pat. No. 4,929,655 describes a process for preparing an aqueous dispersion of hydrophilic polymer in an aqueous saline solution in the presence of a dispersant. The reaction time is 10 hours.
U.S. Pat. No. 5,605,970 repeats the Hymo process, incorporating into the dispersion a hydrophobic monomer. The polymerization times are between 18 and 24 hours.
Patent EP 1 040 141 also proposes a process for preparing aqueous dispersions for which the polymerization times are between 10 and 20 hours.
One of the problems of all these prior art processes is the very long preparation time, between 6 and 24 hours. This technological limit is explained by the need to perform the polymerization at reasonable temperatures generally of between 10 and 80° C., usually between 20 and 60° C. or, more precisely, between 30 and 50° C. approximately. Failing this, the viscosity of the reaction medium increases such that it irreversibly gels.
Cooling of the reaction medium is essential. It must be efficient and homogeneous, maintaining a temperature that is low enough not to bring about viscosification of the medium, which would be detrimental to the production of an aqueous dispersion.
The problem that the invention proposes to solve is that of reducing the preparation time for aqueous dispersions of hydrophilic polymer.